In many system applications, for example in the cellular mobile radio telephone communications field, it is desirable to provide a radio receiver having more than one receiving path for receiving a radio signal. For example, when a radio frequency signal is transmitted on more than one frequency or when two or more receiving antennas are used, the output signals received by a receiver on the plural receiving paths may be combined in order to optimally detect the desired information in the received signal. Oftentimes, the signal quality and signal strength among each of the received signals vary to some degree. By either combining the plural channel signals or selecting the channel having the optimal received signal, higher quality reception is achieved.
There are two techniques for handling signals received on plural channels. The first technique is selection diversity, and as its name suggests, a single, best signal processing channel is selected from a plurality of received signals. In the second technique, diversity combining, different signals received over the plural channels are processed in parallel and combined in some optimum way. Diversity combining is further divided into two categories according to whether the processed signals are combined before demodulation or after demodulation. If the signals are combined before demodulation, a control device is required to insure that the processed signals are combined in phase. In contrast, if the signals are combined after demodulation, the combination is generally insensitive to phase differences. Thus, predetect combination diversity refers to signal combination before demodulation, and postdetect combination diversity refers to signal combination after demodulation.
In a conventional, superheterodyne receiver, as illustrated in FIG. 1, a received signal first passes through a bandpass filter 10 before it is amplified in a RF amplifier 12. The bandpass filter 10 filters out-of-band signals that may saturate the RF amplifier 12. In other words, the filter 10 insures that only the desired signal components are amplified. After amplification, the output signal produced by the amplifier 12 passes through a second bandpass filter 14. The bandpass filter 14 filters out any remaining out-of-band signals that were not completely suppressed by the bandpass filter 10. In addition, the bandpass filter 14 reduces noise and interference at other frequencies to which the mixer may exhibit undesired responses. The output signal from the second bandpass filter 14 is received by a frequency mixer 16. By mixing a signal from a local oscillator 18 with the filter output signal, the mixer 16 converts the received frequency into an intermediate frequency suitable for further conventional receiver processing, such as demodulation, as indicated by the demodulator 20.
Unfortunately, the two bandpass filters 10 and 14 cause some loss of desired signal energy. Consequently, there is a compromise in radio receiver design between sensitivity to the desired signal components and insensitivity to interfering signal components on other frequencies as well as to background noise. In situations where interference and/or noise signal components are absent or at a negligible level, it would be advantageous to eliminate the decrease in sensitivity to the desired signal caused by the bandpass filters 10 and 14. Accordingly, a primary object of the present invention is to increase the sensitivity of the radio receiving apparatus when interference and/or noise signals are either absent or at a negligible level by bypassing a receiving path having one or more of the bandpass filters 10 and 14 and receiving signals on another path having fewer or no filters.